1. Field of the Invention
The present invention relates to a speaker driving circuit suitably used for driving a relatively small speaker such as a speaker incorporated in a television receiver.
2. Description of the Related Art
In a television receiver, for example, a relatively small speaker is used because the space is limited so that bass of an ultralow region cannot be reproduced well generally. In addition, human auditory characteristics vary according to levels of sound, and have a tendency as follows. As a sound level becomes low, sensitivity for an ultralow range is also lowered. Accordingly, when a sound level is low, there is a problem that it is difficult to hear ultralow bass.
In order to solve the above problem that it is hard to hear ultralow bass with its sound level being low, there has hitherto been known a method of using a so-called loudness control circuit, which makes frequency characteristics of a speaker driving circuit variable corresponding to a reproducing level of a sound level. By the loudness control circuit, frequency characteristics of a speaker driving circuit is changed to be relatively flat when a reproducing level is high, and a low range of frequency characteristics of the speaker driving circuit is made to boost when a reproducing level is low, When the reproducing level is low, a high range is also emphasized by the loudness control circuit. FIG. 5 is a circuit diagram showing an example of a loudness control circuit.
In explaining according to FIG. 5, reference numeral 1 in FIG. 5 denotes an audio signal input terminal for receiving an audio signal, this audio signal input terminal 1 is connected to a one end of a coupling capacitor 2, while the other end of this coupling capacitor 2 is grounded by way of a series circuit of capacitors 3, 4 and a resistor 5, and a connection mid point of the coupling capacitor 2 and capacitor 3 is grounded through a variable resistor 6, and a grounding mid point of the capacitors 3 and 4 is connected to an intermediate point of a resistor element 6a of this variable resistor 6.
An audio signal obtained at a movable element 6b of this variable resistor 6 is supplied into a power amplifier 8 through a coupling capacitor 7, and an audio signal obtained at the output side of this power amplifier 8 is supplied into a voice coil of a speaker 9, and the diaphragm of this speaker 9 is oscillated to emit a sound.
As shown in FIG. 6, frequency characteristics of an audio signal supplied from the power amplifier 8 to the speaker 9 is flat from the ultralow range to the high range when the reproducing level is high. When the reproducing level is low, the ultralow range and the high range is boosted. Therefore, irrespective of a high or low level of reproduction, ultralow range sound can be heard. Furthermore, the frequency characteristics extend to the ultralow range so that bass of high quality can be obtained without phase loss of the signal.
However, with a relatively small speaker such as a speaker incorporated in a television receiver, when the level of audio signal is raised and the speaker 9 is driven by the ultralow range signal of high level, the diaphragm of the speaker 9 itself does oscillates, but cannot drive the air sufficiently. So, adverse effects are caused on the sound quality.
The present invention is devised in the light of the above problems, and it is an object thereof to avoid adverse effects on the sound quality even if the level of the audio signal is raised.
The present invention presents a speaker driving circuit comprising a volume adjusting means for adjusting an output level of an audio signal supplied to a speaker and an ultralow bass emphasizing circuit frequency characteristics of which is variable corresponding to an adjusted state of the volume adjusting means, which further comprises level detecting means for detecting an adjusted state of the volume adjusting means and ultralow bass component reducing means for, when it is detected by the level detecting means that the volume adjusting means is adjusted to make an output level of the audio signal become higher than a specified level, reducing an ultralow bass component of the audio signal.
According to the present invention, as far as the audio signal is below the medium level, the speaker is driven by a signal extended in the frequency characteristic to the ultralow range, the bass of high quality is obtained without phase loss, but when the audio signal is above the medium level, since the ultralow range is cut off by the high-pass filter, clip or the like does not occur, and the diaphragm of the speaker does not oscillate in the ultraslow range, so that there is no adverse effect on the sound quality.